1. Field of the Invention
The present invention relates generally to a variable optical filter used, for example, in an optical communication system, and more particularly, to a variable optical filter that has two phase difference elements disposed at the front and back, respectively, of a variable Faraday rotator in an optical path and sets the optic axes of the two phase difference elements in different azimuths to ensure improved linearity (linearity when transmittance is expressed in logarithmic form) over a wide operational slope range.
2. Description of the Related Art
In the wavelength division multiplexing (WDM) communication, one or a plurality of optical amplifiers such as erbium-doped fiber amplifiers (EDFA) are incorporated in each repeater to amplify an attenuated signal for transmission over a long distance. In this case, the amplification vs. wavelength characteristic of the EDFA affects the communication quality. Despite the correction made on each of the EDFAs using a fixed optical gain equalizer, the EDFAs develop a slope in the wavelength characteristic due to their secular change and variations in the input signal light. If this slope accumulates in a plurality of the concatenated EDFAs, the optical signal-to-noise deteriorates in channels with a large loss.
A variable optical filter is used to correct this slope. The related art includes a variable optical filter having a first polarizer, a phase difference element, a variable Faraday rotator and a second polarizer arranged in this order along the optical axis. See Japanese Patent Application Laid-open Publication No. 11-212044.
If the wavelength slope characteristic is compensated for with such a variable optical filter, the transmittance has dependency on the rotation angle of the variable Faraday rotator and the wavelength, and the wavelength characteristic of the phase difference elements changes like a trigonometric function. In general, however, the slope of the EDFAs is known to vary linearly when the wavelength characteristic thereof is expressed in logarithmic form. Despite the demand to correct this slope with the utmost accuracy, the related art has failed to sufficiently meet this demand.